The aim of this application is an understanding of the mechanism of action of interferon at the molecular level. Three major lines of research are proposed: 1) Studies on the interferon receptor, carried out with Alpha interferon produced in E. coli by recombinant DNA technology and labeled by iodination. The interferon receptor of human cells will be characterized and the changes which may occur in the receptor upon binding of interferon will be studied. The long term objective of this line of research is an understanding of the mechanism which transmits specific instructions from the cell surface to the nucleus in order to activate the transcription of specific genes. 2) Studies of antiviral mechanisms at the molecular level. Interferon induces the synthesis of two enzymes, which are activated by double-stranded RNA, a protein kinase and a (2-5)oligoadenylate polymerase. The involvement of these enzymes in the antiviral activity toward specific viruses will be established by measuring the amount of (2-5)oligo(A) present in interferon-treated cells infected with different viruses and by following the fate of viral mRNA in these cells. Other experiments are proposed to assess the role of the protein kinase in the antiviral state and a working hypothesis is presented, which accounts for the selective inhibition of viral mRNA translation as a consequence of a localized inactivation of initiation factor eIF-2. 3) Studies on a novel effect of interferon: the increase in the S-adenosylhomocysteine concentration in interferon-treated cells. This results in an increased ratio of S-adenosylhomocysteine/S-adenosylmethionine and in an inhibition of some transmethylases. Experiments are proposed to test whether this is the explanation for the synthesis of vesicular stomatitis virus mRNA unmethylated in the guanosine of the cap. This mRNA is inactive in protein synthesis. The observations made in HeLa cells will be extended to other human and murine cells. Possible causes for the increase in S-adenosylhomocysteine will be investigated by measuring the activity of enzymes which synthesize and degrade this compound in extracts of interferon treated and control cells. The long term objective of these lines of research is an understanding of the antiviral action of interferon at the molecular level.